Life curves for new and thermally aged oil-impregnated paper insulation

The life-time of oil-impregnated (OI) insulation is mainly governed by two aging mechanism, i.e. electrical and thermal. The electrical-life law takes the form of inverse power law. In this case the life is inversely proportional to the applied electrical stress. Due to continuous voltage application, the insulation loses the dielectric strength. The process of thermal aging of OI insulation is described by Arrhenius law. The law specifies that the rate of thermal aging is exponentially dependent on applied temperature. Due to thermal aging, the structure of the insulation is changed because of chemical decomposition due to pyrolisys, hydrolisys and oxidation. The changes of physical structure result in change of electrical properties of insulation. In order to determine the life-time of the component, the electrical endurance tests are performed at several voltage levels. By the tests performance the time-to-breakdown at given voltage levels are obtained and the life-line of the insulation together with accompanying parameters can be found. However, the mentioned procedure takes only electric aging into account and the thermal aging is neglected here. Therefore, it is interesting how thermal aging influences changes of electrical life. In this paper, the first results of the experiments investigating influence of thermal aging on the electrical life of the OI insulation and its practical implications will be discussed.